Many network providers of telecommunication services, such as AT&T, now utilize Synchronous Optical NETwork (SONET) rings interconnected in a network for transporting optically formatted traffic. A typical SONET ring subnetwork comprises a plurality of nodes, each linked to its two neighbors by an optical fiber service path and protection path in a ring-like arrangement. At each individual node, an Add-Drop Multiplexer (ADM) typically routes traffic onto, and off the ring subnetwork to permit traffic to pass on the ring between nodes. In practice, inter-ring traffic received at a node on one ring is coupled by a Digital Cross-Connect System (DCS) for routing on a second ring subnetwork to its ultimate destination.
During normal operation, optically formatted traffic within a ring passes from one node to another via the optical fiber service path. Should the optical fiber service path connecting a pair of nodes become unavailable to carry traffic, due to a fiber cut, for example, the traffic that would otherwise pass on the service path passes on the protection path. U.S. Pat. No. 5,182,744, issued on Jun. 26, 1993, in the name of James Askew et al., and assigned to AT&T Bell Laboratories (incorporated by reference herein) describes a technique for restoring optically formatted traffic in a mesh network in case of a failure of the optical service path between nodes. The Askew et al. technique can be utilized in a ring subnetwork to restore traffic in case of a "single" ring failure, i.e., the failure of a optical service path or a node linked to such a path.
In addition to suffering single failures as discussed above, a SONET ring subnetwork can also suffer, for example, a "double" ring failure. A double ring failure occurs when optical service and protection paths on a given span in a SONET ring fail, due to a cable break for example, and a "non-associated" protection path (i.e., a protection path on a span different from the span with the cable break) also fails. A double ring failure is generally not restorable via the ring itself, giving rise to a service outage which is costly from both a revenue and customer satisfaction perspective.
Thus, there is a need for a technique for restoring service in an optical subnetwork, such as a SONET ring subnetwork, in case of a double failure.